A planar light source device is incorporated in the liquid-crystal display devices used in, for example, personal computers, cell phones, and so forth, in order to respond to demands for greater thinness, lighter weight, greater labor savings, and higher definition. In addition, with the goal of functioning to uniformly and efficiently guide the incident light to the liquid-crystal display side, this planar light source device is provided with a flat plate-shaped light guide plate or a light guide plate that has a wedge-shaped cross section in which one surface has a uniformly sloped surface. In some instances, a peak-and-valley pattern is also formed in the surface of the light guide plate in order to provide a light scattering function.
Such light guide plates are obtained by the injection molding of a thermoplastic resin, and the aforementioned peak-and-valley pattern is generated by transfer from a peak-and-valley region formed in the surface of an insert. Light guide plates have in the past been molded from a resin material such as polymethyl methacrylate (PMMA); however, of late a conversion has been underway to the more highly heat-resistant polycarbonate resin materials due to the trends of demand for display devices that render sharper images and the higher temperatures within the device caused by the heat produced in proximity to the light source.
Polycarbonate resins exhibit excellent mechanical properties, thermal properties, and electrical properties and an excellent weatherability; however, they have a lower light transmittance than PMMA or the like and the problem thus arises of a lower brightness when a planar light source assembly is constructed of a light source and a polycarbonate resin light guide plate. In addition, there has lately been demand for a small chromaticity difference between the incident light area of the light guide plate and locations distant from the incident light area, but a problem here is that polycarbonate resin more readily undergoes yellowing than does PMMA resin.
A method is proposed in PTL 1 in which the light transmittance and brightness are improved by the addition of an acrylic resin and an alicyclic epoxy; a method is proposed in PTL 2 in which the brightness is improved by modifying the polycarbonate resin terminals and raising the transferability of the peak-and-valley region to the light guide plate; and a method is proposed in PTL 3 in which the brightness is improved by improving this transferability by introducing a copolyester carbonate that has an aliphatic segment.
However, in the case of the method in PTL 1, while the addition of the acrylic resin does bring about a good hue, the light transmittance and brightness cannot be raised due to the appearance of cloudiness. The addition of alicyclic epoxy can improve the transmittance, but a hue-improving effect is not recognized for this. In the case of the methods in PTL 2 and PTL 3, an improvement in the flowability and transferability can be expected, but the problem occurs of a reduction in the heat resistance.
On the other hand, the incorporation of, for example, a polyethylene glycol or a poly(2-methyl)ethylene glycol in thermoplastic resins such as polycarbonate resins is known, and PTL 4 describes a γ-radiation resistant polycarbonate resin that contains same, while PTL 5 describes a thermoplastic resin composition that is provided by incorporating same in, for example, PMMA, and that has an excellent static inhibition and an excellent surface appearance.
PTL 6 proposes that the transmittance and hue be improved through the incorporation of a polyethylene glycol or poly(2-alkyl)ethylene glycol with the formula X—O—[CH(—R)—CH2—O]n—Y (R is a hydrogen atom or a C1-3 alkyl group). Some improvement in the transmittance and yellowing (yellow index: YI) is seen due to the incorporation of polyethylene glycol or poly(2-alkyl)ethylene glycol.
However, the trends toward greater thinness and greater thinness in large sizes have in particular been developing quite rapidly recently with regard to various mobile terminals such as smart phones and tablet form-factor terminals, and an edge configuration, in which light injection into the light guide plate is carried out from a lateral edge, is being adopted rather than the vertical configuration and a satisfactory brightness has come to be required from ultrathin light sources. The current state of such high-end light guide plates is that they do not satisfy the required specifications for transmittance and YI level that are achieved by the above-described prior art.